This invention relates to a radio receiver, and more particularly to a radio receiver which can receive a signal of a desired frequency stably and accurately with a desirable characteristic of suppressing a spurious signal, and further to a radio receiver which can display the received frequency in digits by means of a frequency counter.
Generally, in short wave broadcasting, the carrier frequency uses a very wide frequency band from 3 MHz to 30 MHz, and there are broadcasting stations numbering 4000 to 5000 in the world. The minimum unit of the carrier frequency regulation of these stations is on the order of kilohertz. Therefore, a receiver for receiving a short wave signal is required to receive the desired signal accurately and to continue the receiving stably for a long time. However, conventional short wave radio receivers are not always satisfactory for the above requirements. It is often troublesome to tune the desired station accurately, and even when the receiver is accurately tuned to the desired station there is often a problem of drift of the received frequency.
As one of the methods of improving the accuracy of tuning, there is a method of counting the oscillated frequency of a local oscillator digitally and displaying the numeral of the short wave frequency to be received. In such a short wave receiver, the received frequency should be counted and displayed accurately and stably. In the prior art, for a superheterodyne radio receiver, known to be one of the best methods of improving the display accuracy of the received frequency, the oscillated frequency of the local oscillator is directly counted and corrected with addition or subtraction of the intermediate frequency of e.g. 455 KHz, and then the received frequency is digitally displayed. However, conventionally there is a problem in this method as follows. That is, in receiving the signal of short wave band of 3 to 30 MHz, although the frequency drift of the local oscillation frequency near to the lower end of the above band is comparatively small, for a higher frequency of that band it is difficult to hold stably the desired signal, which is once tuned accurately, for a long time. Further, in the case when a digital displaying device as described above is installed in a radio receiver, it becomes necessary to count the frequency up to a higher frequency. That is, although it depends on the intermediate frequency, in the case of 455 KHz intermediate frequency, it is necessary to count the frequency up to 30.455 KHz. This fact requires a high speed integrated circuit such as TTL for a radio receiver, and it is undesirable because of power consumption thereof and of heat generation in the set and spurious radiation.
For the problem described above, there is a method using a variable frequency oscillator having an oscillation frequency enough lower than the upper limit, e.g. 30 MHz, of the desired receiving frequencies and a reference frequency oscillator such as a crystal oscillator having a stable reference oscillation frequency. The output signals of these two oscillators are mixed and passed through a band pass filter, and then there is provided a desired higher local oscillation frequency which is the sum or difference of these two oscillation frequencies. However, in this method it is very difficult to take out only the desired local oscillation frequency because of difficulty of eliminating all of the spurious signals, i.e. higher harmonics of the two oscillation frequencies and cross modulation signals thereof, at the same time. In addition, there are further disadvantages that such a method requires many band pass filters and that in case of displaying the received frequency by means of a digital counter a high speed presettable frequency counter, which is usually expensive, is required, as described hereinbefore.